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Carburetion Systems

Small Engines

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CarburetionFunctions of the carburetion system are:

To mix the fuel with the proper proportion of air. To vaporize the fuel To deliver the correct amount of the air-fuel

mixture to the cylinder.A carburetor may be defined as:

A device for automatically mixing fuel in the proper proportion with air to produce a combustible gas mixture.

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CarburetionAdditional engine parts closely related

to the carburetor include: The fuel tank, fuel lines, air cleaner, choke,

and speed control devices (governors).

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Venturi and Air FlowA venturi in the the carburetor increases

the velocity of the incoming air. When the air is forced through a

restricted area, it must accelerate in order to maintain the volume of flow.

This is a lot like a narrow space in between to buildings.

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Venturi and Air FlowA venturi is defined as:

An area in an air flow tube of a carburetor that restricts the flow of air through the tube resulting in a high velocity and low pressure at the restricted area.

Venturies may differ in their general configuration from one carburetor to another.

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Venturi and Air FlowAir flow through the venturi area of the

carburetor is due to the movement of the piston.

The pressure in the intake passage is decreased.

Thus creating a low pressure condition in the carburetor venturi area.

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ThrottleTo regulate engine operating speed a

throttle valve is employed to restrict air flow.This is mounted just beyond or above the

venturi.When the throttle valve is fully opened the

air flow is affected very little. When as the throttle is closed the flow of air

is restricted to the intake passage.

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ThrottleThis decreases the speed and power of the

engine.At the same time it allows the pressure in the

venturi area to increase.The difference between the air pressure in

the fuel chamber and the venturi is decreased.

Therefore the movement of fuel through the nozzle is reduced.

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Carburetor TypesAll carburetors used on small engines are

basically the same.Carburetors can be classified into three

kinds: Float Feed Suction Feed Diaphram

The difference between these is the way the fuel is supplied to the fuel chamber.

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Carburetor TypesFloat Feed carburetors locate their fuel

tanks some distance from the carburetor.

The fuel flows either be gravity, or due to the force of a fuel pump, through fuel lines to the lower part of the carburetor.

The position of the float controls the fuel level in the bowl.

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Carburetor TypesSuction Feed Carburetor is very similar to the

float type.The one exception is there is no float to

meter and control the level of fuel in the fuel chamber.

The difference in pressure between the tank and the carburetor throat lifts the fuel up the fuel pipe past the main needle valve and through the discharge holes.

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Carburetor TypesThe diaphram carburetor has a rubber

diaphram exposed to the cylinder intake stroke vacuum on one side and to atmospheric pressure on the other.

The diaphram moves against the inlet needle allowing it to move from its seat.

A spring returns the needle to its seat when the vacuum stops.

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Carburetor AdjustmentsCarburetors need adjustment to meet

climatic conditions of both sea level and temperature variations.

Carburetors should be adjusted so it will not exhaust an unnecessary amount of unburned hydrocarbons or it should not be so lean that it will give unsatisfactory power performance.

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Idle and Main Needle Valve AdjustmentsThe main needle valve adjustment is has many

different names. Power load adjustment, high speed adjustment screw,

main jet, etc…The valve is usually turned by hand or by a

screwdriver.The turning moves the needle’s tapered point in

and out of a seat in the lower part of the nozzle, thus varying the amount of fuel that can be metered.

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Idle and Main Needle Valve AdjustmentsAdjustments of the main needle valve

should be done before adjusting the idle valve.

Turn the main needle until the engine starts to flood.

Then quickly turn the valve in the opposite direction until until the engine starts to miss from the lack of fuel.

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Idle and Main Needle Valve AdjustmentsSomewhere between these points will

be the correct main needle valve setting.

The idle valve should be adjusted next.

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ChokesStarting an engine when it is cold

requires a richer air-fuel mixture than when it is warm.

The choke regulates the pressure in the venturi, thereby increasing the flow of fuel from the discharge nozzles.

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GovernorsThe function of the governor on an

engine is to maintain a desired speed regardless of load.

It is a fixed throttle position, the engine will speed up or slow down depending on the load applied.

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GovernorsThe two most common governor systems

are the air vane type and the mechanical type.

The air vane governor is operated by the force of air currents away from the flywheel fins.

This force and movement of the air on the air vane opens and closes the throttle valve.

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GovernorsMechanical governors work in a manner

similar to the air vane type except centrifugal weights oppose the governor spring rather than an air vane.

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Air Cleaners It is important to prevent dirt and dust

from being cared into the engine through the carburetor.

If the air cleaner is operating improperly dirt will enter the combustion chamber, causing excessive wear of the rings, cylinder walls and other moving parts.

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Air CleanersThere are several different types of air

cleaners used.The oil bath type is the oldest type.All air passes through the oil and oil soaked

mesh before entering the carburetor.Dust and dirt are removed from the air and

accumulates as sediment in the bottom of the oil cup.

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Air CleanersOil saturated type of air cleaner,

contains foam or fine wire mesh which is saturated in oil.

A third type of air cleaner is the dry element type.

Commonly found on larger engines.